I used the CDW waiver on a different card, and it was primary because I did not have car insurance at the time. (Which, incidentally, despite much confusion over the issue, is usually perfectly okay.) I found dealing with the people at the claims management company to be an absolute delight.

I would read the fine print and expect it to be fully honored. Remember, though, that this is not liability insurance, which means that there's a very substantial possibility that your insurance company will become involved anyway.